1. Field of the Invention
This invention concerns a method and an apparatus for liquefying natural gas. In another aspect, the invention concerns an improved driver and compressor configuration for a cascade-type natural gas liquefaction plant.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
With regard to ease of storage, natural gas is frequently transported by pipeline from the source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
There are five key economic drivers that must be considered when designing a natural gas liquefaction plant: 1) capital expense; 2) operating expense; 3) availability; 4) production efficiency; and 5) thermal efficiency. Capital expense and operating expense are common financial criteria used to analyze the economic feasability of a project. However, availability, production efficiency, and thermal efficiency are less generic terms that apply to projects utilizing complex equipment and thermal energy to produce a certain quantity of a product at a certain rate. In the area of natural gas liquefaction, xe2x80x9cavailabilityxe2x80x9d is simply a measure of the amount of time that the plant is online (i.e., producing LNG), without regard to the quantity of LNG being produced while the plant is online. The xe2x80x9cproduction efficiencyxe2x80x9d of an LNG plant is a measure of the time which the plant is online and producing at full design capacity. The xe2x80x9cthermal efficiencyxe2x80x9d of an LNG plant is a measure of the amount of energy it takes to produce a certain quantity of LNG.
The configuration of compressors and mechanical drivers (e.g., gas turbines, steam turbines, electric motors, etc.) in a LNG plant greatly influences the capital expense, operating expense, availability, production efficiency, and thermal efficiency of the plant. Typically, as the number of compressors and drivers in an LNG plant is increased, the availability of the plant also increases due to the ability of the plant to remain online for a larger percentage of time. Such increased availability can be provided through a xe2x80x9ctwo-trains-in-onexe2x80x9d design in which compressors of a refrigeration cycle are connected to the refrigeration cycle in parallel so that if one compressor goes down, the refrigeration cycle can continue to operate at a reduced capacity. One disadvantage of the redundancy required in many xe2x80x9ctwo-trains-in-onexe2x80x9d designs is that the number of compressors and drivers must be increased, thereby increasing the capital expense of the project.
It is also known that the thermal efficiency of a natural gas liquefaction plant can be increased by recovering heat from certain heat-producing operations in the LNG plant and transferring the recovered heat to heat-consuming operations in the plant. However, the added equipment, piping, and construction expense required for heat recovery systems can greatly increase the capital expense of a LNG plant.
Thus, it is readily apparent that a balance between capital expense, operating expense, availability, production efficiency, and thermal efficiency exists for all LNG plant designs. A key to providing an economically competitive LNG plant is to offer a design that employs an optimum balance between capital expense, operating expense, availability, production efficiency, and thermal efficiency.
It is an object of the present invention to provide a novel natural gas liquefaction system having an optimum driver and compressor configuration that minimizes capital and operating expense while maximizing availability, production efficiency, and thermal efficiency.
It is another object of the invention to provide a novel natural gas liquefaction system having a waste heat recovery system that greatly enhances thermal efficiency without adding significantly to capital or operating expense.
It should be noted that the above objects are exemplary and need not all be accomplished by the claimed invention. Other objects and advantages of the invention will be apparent from the written description and drawings.
Accordingly, in one embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) using a first gas turbine to drive a first compressor, thereby compressing a first refrigerant of a first refrigerant cycle; (b) using a second gas turbine to drive a second compressor, thereby compressing the first refrigerant of the first refrigerant cycle; (c) using a first steam turbine to drive a third compressor, thereby compressing a second refrigerant of a second refrigerant cycle; and (d) using a second steam turbine to drive a fourth compressor, thereby compressing the second refrigerant of the second refrigerant cycle.
In another embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) using a first gas turbine to drive a first compressor and a second compressor, thereby compressing a first and a second refrigerant in the first and second compressors respectively; (b) using a second gas turbine to drive a third compressor and a fourth compressor, thereby compressing the first and second refrigerants in the third and fourth compressors respectively; (c) recovering waste heat from at least one of the first and second gas turbines; (d) using at least a portion of the recovered waste heat to help power a first steam turbine; and (e) compressing a third refrigerant in a fifth compressor driven by the first steam turbine.
In still another embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) compressing a first refrigerant in a first compressor driven by a first gas turbine; (b) recovering waste heat from the first gas turbine; (c) using at least a portion of the waste heat recovered from the first gas turbine to help power a first steam turbine; and (d) compressing a second refrigerant in a second compressor driven by the first steam turbine, wherein the second refrigerant comprises in major portion methane.
In yet another embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) compressing a first refrigerant in a first compressor driven by a first turbine, wherein the first refrigerant comprises in major portion a hydrocarbon selected from the group consisting of propane, propylene, and combinations thereof; (b) compressing a second refrigerant in a second compressor driven by the first turbine, wherein the second refrigerant comprises in major portion a hydrocarbon selected from the group consisting of ethane, ethylene, and combinations thereof; (c) using the first refrigerant in a first chiller to cool the natural gas; and (d) using the second refrigerant in a second chiller to cool the natural gas.
In yet still another embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) using at least a portion of the natural gas as a first refrigerant to cool the natural gas; (b) compressing at least a portion of the first refrigerant with a first group of compressors driven by a first steam turbine; and (c) compressing at least a portion of the first refrigerant with a second group of compressors driven by a second steam turbine.
In a further embodiment of the present invention, there is provided an apparatus for liquefying natural gas that employs multiple refrigerants to cool the natural gas in multiple stages. The apparatus comprises first, second, third, fourth, and fifth compressors, first and second gas turbines, a first steam turbine, and a heat recovery system. The first and third compressors are operable to compress a first refrigerant, the second and fourth compressors are operable to compress a second refrigerant, and the fifth compressor is operable to compress a third refrigerant. The first gas turbine drives the first and second compressors, the second gas turbine drives the third and fourth compressors, and the first steam turbine drives the fifth compressor. The heat recovery system is operable to recover waste heat from at least one of the first and second gas turbines and employ the recovered waste heat to help power the first steam turbine.
In a still further embodiment of the present invention, there is provided an apparatus for liquefying natural gas that employs at least a portion of the natural gas as a first refrigerant. The apparatus comprises first and second steam turbines and first and second groups of compressors. The first group of compressors is driven by the first steam turbine and is operable to compress at least a portion of the first refrigerant. The second group of compressors is driven by the second steam turbine and is operable to compress at least a portion of the first refrigerant.